The manufacture of crystalline products is effected by a number of known processes in which crystals are formed in a fluid medium commonly referred to as the mother liquor. In one known process, the mother liquor, which is initially at a relatively high temperature, is cooled rapidly until crystallization commences, at which time the cooling rate is reduced or stopped, to allow the crystals to grow at a slow, controlled rate. Failure to reduce the cooling rate at the appropriate time often results in the formation of very large numbers of small, impure crystals which are unsatisfactory and must be purified by repetition of the crystallization process.
Other crystallization processes are controlled by distilling part of the solvent from the solution at an essentially constant temperature. At the onset of crystallization, the rate of solvent removal is reduced or interrupted by reducing the heat input or by returning all or part of the condensed solvent to the crystallizer.
In some crystallization processes the crystalline products are crystallized simultaneously with the reaction in which the product is formed. This typically is accomplished by dissolving one reactant in a solvent in which the product has limited solubility. The second reactant (either in pure form or dissolved in the same solvent) is then caused to flow into the vessel containing the solution of the first reactant. At the onset of crystallization it is necessary to stop or slow the flow of the second reactant.
In all of the processes referred to above optimum production of the crystalline product depends in large part on the detection of the time at which nucleation commences so that the process under way can be appropriately modified. Heretofore the approximate commencement of nucleation has been determined by the inefficient and labor intensive method of visually monitoring the liquor in its vessel, or by the use of one of two instrumental methods.
One of the instrumental methods is dependent on the detection of an exotherm accompanying the crystallization and was found to be impractical when dealing with commercial scale production of the product. The second instrumental method was based on the determination of differences of refractive indices and also proved to be ineffective in terms of both sensitivity and durability.
The method according to the invention relies upon the detection of light scattered by the presence of crystals in a fluid medium having a refractive index different from that of the crystals.
The same principles upon which the method is based are applicable to the detection of substances other than crystals. Thus the method has utility in a variety of applications.